p-Aminophenol (PAP) is a well-known and very useful industrial chemical. It is used as an intermediate in the production of pharmaceuticals such as paracetamol, in the production of dyestuffs such as sulfur dyes and in making photographic chemicals.
Conventionally, PAP is prepared by hydrolysing p-nitrochlorobenzene to p-nitrophenol. Then hydrogenation of nitrophenol to PAP is carried out using Fe/HCl catalyst. In this multi-step process, quantity of iron (catalyst precursor) required is quite large, subsequently the production of iron sludge is very large, posing a serious effluent problem. The work up of reaction crude is cumbersome. The quantity of iron used is very important for the faster reduction rate.
An important commercial process for the preparation of p-aminophenol involves the catalytic hydrogenation of nitrobenzene in a strongly acidic medium utilising supported platinum based catalysts. In this process, phenylhydroxylamine (PHA) is first formed and this intermediate immediately rearranges in the presence of acid medium to PAP. Other by-products formed are aniline and o-aminophenol.
The second step involves the rearrangement of phenylhydroxylamine to PAP in an acidic medium. In conventional processes, concentrated sulphuric acid is used. In actual practice, both these steps are carried out in a single reactor. The reaction mixture consists of both aqueous as well as organic phases.
In prior art, Brenner (U.S. Pat. No. 3,383,416, 1969) observed that the interruption of the hydrogenation step before all nitrobenzene is consumed helps in the suspension of the catalyst in the nitrobenzene layer. This is advantageous since the aqueous reaction mixture is immiscible with the nitrobenzene layer and the aqueous layer containing PAP, aniline and other minor by-products is readily separated from the catalyst-nitrobenzene layer by decantation. The PAP is then recovered from the aqueous layer and further purified.
Hydrogenation of nitrobenzene to PAP in acetic acid containing CF.sub.3 SO.sub.3 H using Pt/C catalyst is reported (EP 5,58,369, 1992). Lower yield and selectivity are obtained when H.sub.3 PO.sub.4 is used instead of CF.sub.3 SO.sub.3 H in acetic acid medium.
Greco (U.S. Pat. No. 3,953,509, 1976) reported the use of molybdenum sulphide on carbon catalyst for the hydrogenation of nitrobenzene to PAP. Dunn (U.S. Pat. No. 4,264,529, 1981) has reported the use of platinum on (--alumina for the hydrogenation of nitrobenzene to yield PAP. Gaskey and Chapman (U.S. Pat. No. 4,415,753, 1983) and (U.S. Pat. No. 4,571,437, 1986) have suggested low temperature hydrogenation in the presence of a modified catalyst system containing a sulphur compound and the rearrangement step in a separate vessel. Recently, a process has been described by Landsheidt et al. (U.S. Pat. No. 5,302,742, 1994) in which hydrogenation of substituted nitrobenzene (p-position vacant) is carried out in the presence of a co-solvent (in acidic medium with 5% Pt/C and an acylating agent to give corresponding N-acylated p-amino phenols.
Rylander et al. (U.S. Pat. No. 3,715,397, 1973) disclosed a process for the preparation of PAP by catalytic hydrogenation of nitrobenzene in a sulphuric acid medium in the presence of dimethyl sulphoxide, using platinum oxide catalyst.
Generally, nitrobenzene, when first reduced to form PHA, can be further reduced to form aniline. In the presence of acid, PHA undergoes rearrangement to form PAP. As noted, after the formation of PHA, there are two pathways leading to the final products PAP and aniline.
In accordance with this invention, it has now been discovered that the solid acid can be used as an alternative for mineral acid, in the rearrangement of PHA to PAP. The hydrogenation of nitrobenzene is complete and PAP is formed along with aniline.